<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title>CSS基本语法</title>
    <style>
       div.opacity{
           width:200px;
           height: 200px;
           background-color: dodgerblue;
           opacity:0.9;
       }
    </style>
</head>

<body>
    <div class="opacity">
            opacity
    </div>
</body>

</html>